372
edits
Changes
→What is the oxygen concentration in the diluted CPAP circuit?
The equation to calculate concentration from flow is a simple mass balance:<br>
(oxygen flow rate (litres) at 100% O2) + (CPAP air volume (litres) at 21% O2)) / Total air volume = oxygen concentration Percent in circuit <br>Note that the design leak rate for any mask at a given pressure does not increase with an oxygen bleed, so CPAP air volume (Vcpap) must be reduced by the amount of flow from the oxygen generator in L/min, which gives us the following: '''Determination of FiO2 (fraction of inspired oxygen) with CPAP:'''FiO2=((Oxygen Flow L/min x 100%) + (Vcpap L/min - Oxygen Flow L/min) x 21% O2)) / Vcpap
therefore:<br>
'''Scenario 1'''
at 1-litres/minute oxygen addition at 100% and assuming a total CPAP flow Vcpap of 30 L/min:<br>(1-L/min x 100% O2) + (30-1 L/min x 21% O2) = 730709/3130-L/min = '''23.5563% Oxygen'''.<br><br>
'''Scenario 2'''
at 3-litre/minute oxygen at 100% and assuming a total CPAP flow of 30 L/min:<br>
(3-L/min x 100%O2) + (30-3 L/min x 21% O2) = 930867/33 30 L/min = '''28.189% oxygen'''.
Any variable can be changed based on available data. If the flow rate is 40 L/min from the CPAP, the oxygen concentrations will be lower. If leaks are present or pressure results in a higher average vent-rate, the oxygen concentrations will also be lower. If a mask with less vent rate is used, then concentrations of oxygen will increase.
Donate to Apnea Board